Release 2019.6

Nothing major in this release, but we have some
bigger stuff outstanding and ready to merge, so I
want to get this release out so that work will
have time to stabilize.

A few build/CI fixes.  A new progress API which
will be used by flatpak (and can be used by others).
Finally, we also avoid reordering kernel arguments.

Thanks to all contributors!

```
git shortlog --no-merges v2019.5..
Alex Kiernan (5):
      tests/core: Fallback to en_US.UTF-8 locale
      tests: Handle EPIPE failures when head terminates
      tests/core: Assume C.UTF-8 if locale isn't found
      tests: Avoid musl failure with `cp -a`
      build: create tests directory for split builds

Colin Walters (6):
      Post-release version bump
      lib/keyfile: Treat "group not found" the same as "key not found"
      Bump libglnx
      tests/repo-finder: Run realpath() on /tmp
      pull: Add support for basic auth
      Release 2019.6

Philip Chimento (2):
      Bump version in symbols file
      libostree: Add ostree_async_progress_copy_state()

Ricardo Salveti (1):
      Makefile: declare ostree_boot_SCRIPTS and append values

Robert Fairley (1):
      lib/kernel-args: Store kernel args as key/value entries

Sam Thursfield (1):
      README.md: Tweak text about BuildStream

Stefan Agner (1):
      Avoid race condition when building outside of source tree
```

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